Indeed, 4,4'-dihydroxybiphenyl is an intermediate which is currently sought in the industry of synthetic materials such as polyesters, polyepoxides and polyurethanes, as an antioxidant in resins or in the dyestuff industry.
The preparation of hydroxybiphenyls by the hydrolysis of halobiphenyls using a copper catalyst, such as, for example, copper sulfate (according to U.S. Pat. No. 4,475,000) or copper oxide (according to U.S. Pat. No. 4,340,768) in an aqueous basic medium is known.
Many processes for the preparation of hydroxyaryl compounds, which are carried out in the gaseous phase, i.e. at a temperature above 300.degree. C., in the presence of a copper-based catalyst and a rare earth phosphate-based catalyst are also known. Thus, a process for the preparation of hydroxyaryl compounds, the aryl group of which contains one or more benzene rings, starting with equivalent halogenated compounds, by hydrolysis in the presence of a catalyst based on lanthanum phosphate or cerium phosphate and copper, which are jointly precipitated in the form of an inorganic polyphosphate, is described in U.S. Pat. No. 3,752,878. This reaction is carried out as a two-phase reaction, the catalyst being in the solid form, and the halogenated compound to be hydrolyzed being in the gaseous form, because the reaction is carried out at temperatures above about 300.degree. C. The degree of conversion of the starting compound is low and the reaction conditions are too severe for them to be employed on an industrial scale.
The hydrolysis monocyclic aromatic halides at a lower temperature is shown in an abstract of Japanese Pat. No. 69/17372, in which the hydrolysis is carried out at approximately 200.degree. C., in the presence of copper derivatives having the oxidation state two, such as copper sulfate, copper dibromide and copper diacetate. The duration of the hydrolysis reaction is particularly long, and sometimes reaches 10 hours for a yield not exceeding 25%. Therefore, this method cannot be adopted on an industrial scale.
Thus, for many years the industry has been in search of a process for the preparation of hydroxybiphenyls starting with halobiphenyls, which process is economically profitable and not dangerous.